Walk. A lot. Everywhere in fact. Three or four miles at a brisk pace twice a day, and a long strenuous hike up and down hills, over boulders, etc... I liked to walk my dog and we'd do maybe 6-8 miles per day, sometimes more. We would walk very rapidly for 100 steps then sprint for the next 100. You can cover lots of ground that way and never breathe too hard.